Amazon Smart Thermostat Stuck Fan Relay or G-Circuit Short

The fan runs continuously even when the thermostat is set to Auto and no heating or cooling is requested.

This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.

If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.

Possible Causes

G wire shorted to R at furnace or in cable, furnace control board fan relay stuck closed, thermostat fan output stuck ON.

Safety: Turn off furnace/air handler power at the breaker.

Check thermostat settings: Confirm the fan mode is set to Auto, not On.
Inspect G wiring: At both the thermostat and the furnace, ensure the G wire is not touching R or other terminals.
Test board vs. thermostat: With power off, disconnect the G wire from the furnace control board. Restore power. If the fan still runs, the furnace control board relay is stuck. If the fan stops, the issue is in the thermostat or wiring.

Replace the furnace control board if its fan relay is stuck, or replace the thermostat if its G output is stuck ON and the wiring is good.

Hard 4/5

Furnace Control Board or Amazon Smart Thermostat Main Control Assembly

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